Blu-ray Disc, or BD, is an optical disc that uses state-of-the-art blue-violet laser technology to enable consumers to record high-definition TV broadcasting. Developed by the "Blu-ray Disc Founders" group, these companies include Hitachi, LG, Matsushita, Pioneer, Philips, Samsung, Sharp, Sony, and Thomson. Although Blu-ray Disc was primarily designed to be a "consumer high definition video recording format", its very high storage capacities and high-speed data transfer rates also make Blu-ray Disc suitable for storage libraries and ultimately other applications.

A Blu-ray Disc has the same physical size as a DVD (12cm) but has higher data and track densities that give it between roughly three to six times the storage capacity of a standard 4.7GB DVD-R. This feat is made possible using a 405 nm (405 billionth of a meter) blue-violet laser, actually violet-purple, (see figure 1) and an optical pickup head with a 0.85 NA (numerical aperture) lens. Because a blue-violet light laser has a shorter wavelength (405 nm) than the red light (650 nm) used in CD and DVD systems, it allows the laser beam to make a smaller spot on the disc surface. With each bit of data taking up less space on the disc, more data can be stored on a 4.7-inch disc.

A distinct feature of the Blu-ray Disc is the position of the recording layer within the disc. For DVD, the recording layer is sandwiched between two 0.6-mm thick layers of polycarbonate plastic. But the recording layer in a Blu-ray Disc sits near the surface of a 1.1-mm thick plastic substrate and is protected by a mere 0.1-mm thin cover layer. This not only allows for better disc readout, as the laser does not have to travel far to the point of focus, but serves to increase the recording density because the laser can be more narrowly focused by the larger 0.85 NA lens aperture (see figure 1 above). In addition, it minimizes tilt problems associated with substrate stresses which can occur during the injection molding process used to produce them. This can cause the laser light to split into two separate beams, refer to as birefringence, and if it becomes excessive, the drive cannot read data reliably from the disc.

But having the recording layer closer to the surface has its disadvantages as it leaves the disc exposed to accidental abrasions, dust, and fingerprint marks. To overcome this, a specially formulated protective hard-coat is applied on top of the cover layer. This protective coat is hard enough to prevent accidental abrasions and also allows for dust and fingerprints to be removed from the disc by simply wiping.

High-definition video (720p or 1080i) requires five times the recording capacity of standard definition video (480i). The actual HDTV transmission is based on a 19.4 Mbps (Mega bits per second) digital data stream but the maximum data transfer for DVD is about 10 Mbps. Thus, there’s simply not enough bandwidth to put an HDTV program on a recordable DVD format. To achieve the density necessary to put this amount of data on a single-sided 12 cm optical disc, the size of the spots burned into the disc need to be smaller. In addition, the high-definition video will need to undergo compression to be able to store this high-definition picture.

The Blu-ray Disc format employs MPEG-2, Video Codec 9 (VC-1 based on the Windows Media 9 format), or H.264/MPEG-4 AVC video compression techniques in order to lower the data rate (i.e., use less digital data) of the high-definition video. The excellent efficiency of the latest MPEG-4 AVC and VC-1 codec allow image data to be compressed to about one-third the size achieved by MPEG-2 and replay of high quality digital images.

But regardless of the codec utilized, the idea is to compress enough to decrease the bit rate to data capacity levels while trying to preserve the high-definition picture quality to a reasonable standard. This allows the high-definition signal to be recorded without excessive compression, preserving the detail of the original high-definition picture. The audio formats for BD extend beyond the current DVD specifications to include every type of audio codec available. Though, at this writing, the more robust DTS Digital Surround had been selected as the audio technology of choice for Blu-ray Disc (BD).

Blu-ray Disc may have three single-sided single-layer structure capacities of 23.3GB, 25GB, 27GB, and three single-sided dual-layer structure capacities of 46.6GB, 50GB and 54GB (without flipping sides) compared to current DVDs which can hold between 4.7GB and 9.4GB of data. However, recent press announcements had circulated that a "Hybrid" disc was being proposed by the Blu-ray Disc Founders that would be comprised of a double-sided single-layer containing an 8.5GB capacity standard-definition and a 25GB capacity high-definition version.

Blu-ray Disc is a totally new and radical optical storage medium requiring retooling and or construction of new disc manufacturing and replication plants. Thus, Blu-ray Disc does not share compatibility with current DVD technologies.